" "Being" is used in this sentence because the speaker is referring to a state that was ongoing. As for your other sentence, "It is a great pleasure to introduce our company being membership agent since or years and focused entirely on buy/sell/rent/investment of golf club" it is not coherent at all and strongly needs a rewrite in order to state its message more clearly. In your other sentence, "It is a great pleasure to introduce our company" is coherent but everything after it ("being a membership for years and has been focusing entirely on buying, selling and renting investment of golf club") is also needs to be rewritten more coherently.
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